1. Field of the Invention
The invention generally concerns an improved technique for communicating control information to control the operation of an electric lamp, such as for dimming. The invention also relates to a wall controller and a ballast implementing the improved communication method.
2. Description of the Prior Art
Electronic dimming ballasts are commercially available in which dimming of gas discharge lamps, typically fluorescent lamps, is responsive to phase angle control of the AC power line input. Phase angle control involves the clipping of a portion of each half cycle of the AC sinusoidal power line voltage. A common type of phase angle controller, known generally as a forward phase dimmer, clips or blocks a portion of each half cycle immediately after the zero crossing. An example of a forward phase dimmer is the well known triac dimmer. Another type is the reverse phase dimmer, commonly known as an electronic dimmer, which passes the portion of the half-cycle immediately after the zero crossing and blocks the portion of the half cycle before the zero-crossing. In both types, the portion or angle of the half cycle which is blocked is adjustable.
Phase angle dimming in which the phase angle information is supplied by an additional wire separate from the power line inputs powering the ballast are known, for example, from JP-116698, U.S. 4,797,599, and DGM 9014982. Such ballasts are inconvenient in that their installation requires the running of the additional, dim-signal-carrying wire between the controller, which is typically mounted in a wall, and the ballast mounted in the fluorescent lamp fixture in the ceiling. This results in considerable labor costs and is an impediment to market acceptance.
Lamp ballasts are also known which are two-wire devices in which the phase cut signal is not separate from the mains supply but is carried by the hot power line input. These are more attractive, from an installation standpoint, than the threewire devices and are known inter alia from U.S. Pat. No. 4,392,086 (Ide et al), U.S. Pat. No. 5,192,896 (Qin), U.S. Pat. No. 4,866,350 (Counts), U.S. Pat. Nos. 4,449,897 (Sairanen), and 5,101,142 (Chatfield).
A disadvantage of each of the above two-wire ballasts is that they each employ voltage control dimming, i.e. the light output of the lamps is effected through variation in the average line voltage, through control of the conduction angle by the triac or electronic dimmer, which in turn changes the ballast bus voltage feeding the ballast's inverter. The dimming level is inversely proportional to the phase angle blocked or cut from the mains voltage. No phase cut corresponds to a maximum light level while increased dimming (lower light level) corresponds to more phase cut. One adverse consequence is that in order to maintain the light level below 100%, the power line voltage must be maintained in a modified state, i.e. with every cycle having a phase cut portion. This adversely effects the performance of the ballast in several ways. First, it lowers the power factor of the ballast during dimming. Secondly, it increases total harmonic distortion (THD). Third, it increases stress on the circuit components. Fourth, it increases EMI, making it more difficult to meet known standards, especially for consumer use.
Another disadvantage is lamp flicker. When the input conduction angle of the line voltage from the phase angle dimmer falls below a certain value, the preconditioner may not be able to draw enough power to maintain the bus voltage at the required level while supplying power to the lamp driver circuitry. If the ballast is left to operate in this mode, the lamp cycles on and off, resulting in severe flicker. Still another disadvantage is limited dimming range. For example, in U.S. Pat. No. 5,101,142 the effective dimming range is limited to conduction angles less than 90 because at higher angles the output voltage of the preconditioner stays the same. The dimming range is limited at the low end to conduction angles at which the resulting bus voltage is high enough to maintain the compliance voltage sufficiently high so that the lamps don't extinguish. Yet another deficiency is the quality of light regulation. In response to a triac dimming input, the ballast of U.S. Pat. No. 5,101,142 provides a dimming control signal, which varies inverter switching frequency, derived from a bus voltage which varies with the input conduction angle. Thus, lamp light output is effected by two varying parameters, the bus voltage and the inverter switching frequency, which inhibits the accuracy of the dimming control.
U.S. Pat. No. 5,068,576 discloses a step dimmer arrangement in which the wall controller completely cuts or reduces the magnitude of an entire half-cycle, so that there is a missing pulse or a pulse of significantly reduced voltage in the rectified DC output of the ballast's rectifier. The time period between successive missing pulses represents a dimming command. For example, time "n" between missing pulses may represent a 70% dim level while time "m" between missing pulses represents a 90% dim level. In the disclosed ballast, the decoder controls the preconditioner circuit to control the DC rail voltage to the inverter, and thereby control the light level of the lamps. A missing pulse will cause a lamp operated by the ballast to flicker. In the disclosed step-dimming ballast, the flicker is not so objectionable as the user expects a significant and abrupt change in the light output of the lamp when switching from one light level, e.g. 90% to the next, e.g. 75%.
However, where a continuous dimming effect is desired, i.e. where the light should be smoothly adjustable in very small increments, the cutting of entire pulses from the mains supply (and the resulting flicker) would be objectionable to the user.
Accordingly, it is an object of the invention to provide an improved communication method especially suited for lighting control which overcomes the above-mentioned disadvantages of the known techniques. It is another object to provide a wall controller and ballast implementing the improved method.